Replacing fs_mgr_read_fstab() with new fs_mgr APIs

The fstab settings of early-mounted partitions (e.g., /vendor)
will be in kernel device tree. Switch to the new API to get the
whole settings with those in device tree:

    fs_mgr_read_fstab_with_dt("/fstab.device")

The original default /fstab.{ro.hardware} might be moved to
/vendor/etc/. or /odm/etc/. Use another new API to get the default
fstab instead of using the hard-coded /fstab.{ro.hardware}.
This API also includes the settings from device tree:

    fs_mgr_read_fstab_default()

Bug: 35811655
Test: boot sailfish
Change-Id: If8361d891e17fa98b407b0e70f9f8984afcbcfe7